Tag Archives: arnold

Wedge Study 3 – Headlight colour temperature

Headlight Colour

At some point in my car project, I am going to want to ‘turn on’ the headlights and create a volumetric look as though there are particles in the environment. So as I searched for a third attribute to test, I figured I would test subtle variances in the colour temperature of the light.

I checked ‘use color temperature’ in the Arnold settings for the spotlight and iterated up in increments of 100 degrees kelvin

5000 degrees Kelvin

5000 degrees Kelvin

5100 degrees Kelvin

5100 degrees Kelvin

5200 degrees Kelvin

5200 degrees Kelvin

5300 degrees Kelvin

5300 degrees Kelvin

5400 degrees Kelvin

5400 degrees Kelvin

5500 degrees Kelvin

5500 degrees Kelvin

5600 degrees Kelvin

5600 degrees Kelvin

5700 degrees Kelvin

5700 degrees Kelvin

5800 degrees Kelvin

5800 degrees Kelvin

5900 degrees Kelvin

5900 degrees Kelvin

 

What I have learned –

I tested the colour spectrum between slightly yellow light at 5000 through netral ‘daylight’ at 5500 and then through to a bluer light at 5900. I would depend on the manufacturer but I think I will just do what feels right for the scene. Now I have a great look up chart.

 

 

Wedge Study 2 – Car wheel rims

How ‘shiny’ do I want the wheel chrome to be? In my test render at maximum shininess, they seemed to be a distraction and drew too much attention to the eye.

I am not working for a car company and matching an established look, rather I am creating something that looks pleasing so its up to me to find the balance. I am testing the ‘roughness’ value on the specular properties starting at zero for maximum shine and going up in increments of .05 –

Specular roughness at 0

Specular roughness at 0

Specular roughness at 0.05

Specular roughness at 0.05

Specular roughness at 0.1

Specular roughness at 0.1

Specular roughness at 0.15

Specular roughness at 0.15

Specular roughness at 0.2

Specular roughness at 0.2

Specular roughness at 0.25

Specular roughness at 0.25

Specular roughness at 0.3

Specular roughness at 0.3

Specular roughness at 0.35

Specular roughness at 0.35

Specular roughness at 0.4

Specular roughness at 0.4

Specular roughness at 0.45

Specular roughness at 0.45

 

What I have learned – 
As the roughness value goes up, the wheels start to move into a flatter, more diffused plastic kind of look. The renderer mimics a surface the has some granular texture which diffuses the light rays and bounces them out at angles. I prefer the lower values and the more chrome like look but for my taste .15 is just enough to break up the harshness and aliased feel you get at 0.

 

Wedge Study 1 – Car windscreen

Below is wedge study for render attributes using the Arnold renderer – I decided to focus on car materials as this will help in look development for a car project I am doing. The car project will be rendered in a HDRI environment for integration with a live action plate but to make it easy to assess the shaders on their own merit, I have simulated studio lighting conditions –

Wedge study 1 – Windscreen glass
One of the materials that was challenging me a bit was the windscreen glass. To simulate different reflection and refraction properties at different grazing angles. Of particular importance is the ‘reflectance at normal’ value in the specular properties so I began changing the values by increments of 0.3 and saving swatches –

reflectance at normal set to 0

reflectance at normal set to 0

reflectance at normal set to 0.03

reflectance at normal set to 0.03

reflectance at normal set to 0.06

reflectance at normal set to 0.06

reflectance at normal set to 0.09

reflectance at normal set to 0.09

This one above is the one that feels about right to me.

reflectance at normal set to 0.12

reflectance at normal set to 0.12

reflectance at normal set to 0.15

reflectance at normal set to 0.15

reflectance at normal set to 0.18

reflectance at normal set to 0.18

reflectance at normal set to 0.21

reflectance at normal set to 0.21

reflectance at normal set to 0.24

reflectance at normal set to 0.24

reflectance at normal set to 0.27

reflectance at normal set to 0.27

What I have learned –
Given that there is a large overhead softbox light, the higher values look really burned out. However, if the value is too low, the glass is too transparent and looks fake.
A value of about .09 seems to look about right for my tastes. This may need to be reduced in a HDR environment where there are sharper things like tree branches reflecting but for now I will call that good.

 

 

Modern Ray-trace renderers

The goal of a modern physically accurate render engine is to calculate indirect light based on the direct illumination hitting surfaces in the scene.

Arnold’s solution to this is to use ‘Monte Carlo’ ray tracing. It renders the image by randomly tracing samples of possible light paths. Repeated sampling of a given pixel will eventually cause the average of the samples to converge on the correct solution, making it very faithful to reality.
‘Monte Carlo’ in a broader computational context, relates to algorithms for repeated random sampling to obtain numerical results. The gambling analogy is the reason it was named after the famous casino.

Arnold is referred to as an ‘unbiased’ renderer – it does not introduce any systematic error, or bias into the radiance approximation.

For performance increases, some renderers use irradiance caches and photon and light map storage to speed up rasterization. Instead, Arnold opts for the ‘brute force’ ray tracing method, recomputing the GI values for every single shaded point seperately and independently from other points. As explained by Eric Haines of ray tracing news ‘there’s only a single version of the model stored, not one for the rasterizer and one for the ray-tracer. This avoids mis-syncs and means that there is minimal pre-computation time. (Haines, E 2010)

This also simplifies the pipeline, infrastructure and user experience.

The philosophy behind this is perhaps best summed up in the Arnold user guide –

‘your time is more valuable than your computer’s time; why spend an extra 30 minutes working with photon mapping or final gather settings, even if it saves 30 minutes render time (and more often than not it doesn’t). That’s still 30 minutes not spent modeling, animating or lighting.’ (mtoa_user_guide.pdf , 2012)
A down-side of this path-tracing method is that it may not consider all possible paths, and subsequently may not consistently handle phenomena like caustics accurately.

As Marcos Fajardo, the chief architect of Arnold admits – ‘one of the only limitations of Arnold is that it does not really do caustics very well. We haven’t bothered to spend our resources optimizing on caustics.’ (Seymour, M 2012)

He goes on to say that in all his years developing for the film industry, accurate caustics is not something that has ever really been a demand from his clients. He also agrees that further optimization is needed with particularly interest in the areas of motion blur and instancing.

References:

Seymour, M 2012 http://www.fxguide.com/featured/the-art-of-rendering/

Haines, E 2010 http://tog.acm.org/resources/RTNews/html/rtnv23n1.html#art3

mtoa_user_guide.pdf https://www.solidangle.com/support/